diff --git a/include/boost/url/grammar/charset.hpp b/include/boost/url/grammar/charset.hpp
index 73de44aa7..16da97d2a 100644
--- a/include/boost/url/grammar/charset.hpp
+++ b/include/boost/url/grammar/charset.hpp
@@ -143,7 +143,7 @@ find_if_not(
 //------------------------------------------------
 
 #ifndef BOOST_URL_DOCS
-namespace detail {
+namespace implementation_defined {
 
 template<class CharSet>
 struct charset_ref
@@ -176,7 +176,7 @@ struct charset_ref
     }
 };
 
-} // detail
+} // implementation_defined
 #endif
 
 /** Return a reference to a character set
@@ -221,11 +221,11 @@ constexpr
 typename std::enable_if<
     is_charset<CharSet>::value &&
     ! std::is_same<CharSet,
-        detail::charset_ref<CharSet> >::value,
-    detail::charset_ref<CharSet> >::type
+        implementation_defined::charset_ref<CharSet> >::value,
+    implementation_defined::charset_ref<CharSet> >::type
 ref(CharSet const& cs) noexcept
 {
-    return detail::charset_ref<
+    return implementation_defined::charset_ref<
         CharSet>{cs};
 }
 #endif